The invention concerns a bright noble metal preparation for application on ceramic/porcelain surfaces, especially tiles, by firing at a minimum temperature of 900° C. The preparation consists of one or more organic noble metal compounds, at least one flux that consists of organometallic compounds, and at least one vehicle.
The invention also concerns a process for producing articles, especially tiles, with decorated ceramic/porcelain surfaces by firing on a bright noble metal preparation.
Finally, the invention concerns two uses of a bright noble metal preparation of the invention.
Bright noble metal preparations for the decoration of glass, ceramics, porcelain, bone china, tiles, or other silicate substrates generally consist of solutions of organic gold, palladium, and platinum compounds in (usually organic) vehicles, synthetic or natural resins, and fluxes, which ensure good adhesion to the given substrate. Specific organometallic compounds, e.g., alcoholates, carboxylates, resinates, or sulforesinates of the elements rhodium, silver, chromium, bismuth, vanadium, silicon, etc., are used as fluxes. During firing, the organic compounds decompose to the corresponding oxides or metals, which causes the metal film to adhere to the substrate.
The bright noble metal preparations usually used for decoration are fired on ceramic, porcelain, tiles, or bone china at firing temperatures of about 780° C. to 900° C., or even as high as 1,000° C., in exceptional cases. Above this firing temperature, the adhering gold coating of bright noble metal preparations is destroyed when the gold coating recrystallizes and forms small crystals, which results in the development of a dull, brown appearance of the noble metal coating, especially in the case of a gold coating.
In recent years, there has been increasing interest in decorating ceramics and porcelain, especially tiles, by firing on color designs and transparent or opaque glass reliefs at high firing temperatures. In the case of porcelain, for example, it is customary to fire high-temperature rapid-firing glazes at about 1,200° C. in a 60-90-minute firing cycle. Tiles are fired with so-called Vetrosa effects mainly at about 980° C. to 1,060° C. These high firing temperatures are necessary to ensure sufficient fusion of the glazes or reliefs with the substrate.
Preparations that are suitable for high firing temperatures for the applications described above have very recently been developed, i.e., about 1,200° C. for porcelain or about 1,000° C. for tile decorations.
EP 0,440,877 B1 describes noble metal preparations that contain nickel and chromium resinate as fluxes and that are suitable for firing temperatures up to about 1,200° C. on porcelain.
DE 3,736,583 also describes bright noble metal preparations that can be fired at 1,250° C. on ceramic ware made of silicate materials. The flux contained in the preparation consists of an organosilicon compound with three and/or four hydrolyzable groups on the silicon atom and other organometallic compounds.
The above-described bright noble metal preparations suitable for high firing temperatures contain rhodium as one of the fluxes. It is clearly emphasized in the literature that the element rhodium is an indispensable component of bright noble metal preparations (see A. Boettcher, Ber. D. Keram. Ges., Vol. 32, p. 175, 1955; S. G. Tumanov, Steklo i Keramika, Vol. 18, p. 26, 1961; A. A. Milgram, J. Electrochem. Soc., Vol. 118, p. 287, 1971; P. A. Levin, Steklo i Keramika, Vol. 3, p. 36, 1968). The element rhodium is necessary during firing to prevent agglomeration and grain growth of the gold particles that are forming from the organic gold compounds. This allows the development of a uniform, reflective, bright noble metal film, especially a bright gold film. The relatively expensive rhodium-containing preparations are especially heat-resistant and stable to firing at temperatures up to 850° C.
DE 4,130,106 C2 describes a noble metal preparation that contains a greatly reduced amount of rhodium. Thorium is recommended as a substitute for the rhodium, but it is unsuitable for decorative preparations due to its radioactivity.